Microwave beam tube

ABSTRACT

The improved microwave tube is a tube such as a klystron wherein bunches of electrons are produced by the input cavity of the tube. These electron bunches are well defined and are produced only when the input signal exceeds a level determined by the tube design. The bunches of electrons are obtained from a field effect electron emitter which forms a portion of one wall of the input cavity of the tube. The device is capable of instant response immediately upon input signal application and results in improved output signal energy and quality.

United States Patent 1191 Shelton et a1.

[ MICROWAVE BEAM TUBE Sept. 13, 1974 [21] Appl. No.: 505,827

[52] US. Cl. 3l5/5.39; 313/309; 313/336 [51] Int. C1. H01J 25/10 [58] Field of Search 313/336, 309; 315/135,

1 Nov. 18, 1975 Denholm et a1. 315/539 X Shelton 313/336 Primary Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or FirmEugene E. Stevens, 111; Jack W. Voigt [5 7] ABSTRACT 1 Claim, 3 Drawing Figures [56] References Cited UNITED STATES PATENTS 3,091,719 5/1963 Dyke et a1. 313/336 x 3,109,123 10/1963 Spencer 313/336 x 3,489,943 1 1970 1366116161 315/539 x US. Patent Nov. 18, 1975 3,921,027

30 IO 30 3O 30 j 20 28 2s Ie- I I II I2 {I I4 22 24 2 K FIG. I

l I I I I BEAM FROM INPUT DRIFT REsoNANT DRIFT I oUTPUT I ELEcTRoN CAVITY I TUBE cAvITY I TUBE I CAVITY I A I I TUBEWITHNO INPUT SIGNAL) I I I I I (CONVENTIONAL TUBE WITH INPUT SIGNAL) I I I I I l I (IMPROVED TUBE I I I WITH NO INPUT I SIGNAL) I I I D I I (IMPROVED TUBE '-J- I I I WITH INPUT SIGNAL) I I I I TYPICAL ELEcTRoN BEAM PATTERN FIG. 3

MICROWAVE BEAM TUBE .DEDICATORY CLAUSE The invention described herein may be manufactured, used. and licensed or for the Government for governmental purposes withoutpayment to us of any royalty thereon. I T

BACKGROUNDOF THE INVE TION In the conventional klystron tube a steady stream of electrons is developed-from an electron gun consisting of a thermionic emitter anda beam acceleration anode. The constant beam is directed thr ough' an inputcavity toward a collector. A signal applied to an input loop causes oscillations in the input cavity, resulting in a varying signal on the sides of the input cavity which causes bunching of the electrons. Additional cavities cause the electrons to be formed into more compact and distinct bu nches Microwave energy is extracted from an output cavity just before the stream of electrons reaches the. collector. The output energy is obtained in essentially the reverse manner that energy is applied to the input cavity. This microwave energy is coupled through anoutput iris and an output window to some load such an an antenna. Tuning diaphragms associated with each cavity allow precise tuning of the respective cavities. I v v It is well established in prior art klystronsthat the source of electrons is completely independent of the radio frequency input circuit. The constant stream of electrons from the electron source is introduced into the klystron buncher (input cavity) wherein the radio frequency field acts. on the beam. The beam isvelocity modulated in the buncher withs ubsequent drift spaces and cavity resonators enhancing the alternating component of the modulated beam by increased bunching as is well established in the art. For the electron velocity to be efficiently modified, the electrons must cross as klystrons are well established in the art. Detailed descriptions are found in publications's'uch as Introduction to Klystron Amplifiers, Varian Associates Publication AEB No. 19 dated April 1963, by Bruce E. Nelson, and KlystronTubes by A. .E. Harrison. McGraw-' Hill Book Company, Incorporated, 1947.

The field effect electron emitter is'an oxide-metal matrix comprised of ordered metal fibers separated by an insulating oxide. The'emitter'may be comprised of several million fibers arran ged'iri parallel for each square centimeter of emitter surface areawi th the ends of the fiber forming the emitter surface. The fiber ends are all of substantially the same diametenwith the distance between adjacent fiber ends beingsub stantially the same. With the field effectelectron emitter, n'o low energy electrons are generated. Advances in field effect electron emitters are set forth in'U.S. Pat. Nos. 3,745,402 and 3,746,905 by Shelton et al. and US. Pat. No. 3,783,325 by Shelton.

SUMMARY OF THE INVENTION The apparatus of the instant invention relates to beam tubes, such as klystrons and similar microwave tubes. The tubes are improved by elimination of the electron gun assembly and the direct emission of electrons within the input cavity. resulting in increased efficiency. reduced size and weight, and instantaneous response to input signals. A field effect electron emitter lies within and forms a portion of the input cavity of the klystron tube. The emitter operation is a function of the input radio frequency signal to the input loop of the cavity. The emitter conducts during alternate half cy cles of the RF input signal, producing electron bunches in well-defined, distinct groups when the input signal exceeds a level determined by the tube design, and re sults in improved output signal energy.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT 1 FIG. 1 discloses a preferred embodiment of a klystron tube in accordance with the teachings of the instant invention. The basic operation is completely different from that of prior art klystrons. In this microwave tube. there is no conventional electron gun and the individual electrons are produced in distinct bunches rather than as a steady electron stream. FIG.'1 discloses an improved klystron tube 10 having an input cavity 12 at the base thereof. Cavity 12 has an input loop 14 coupled through the outer wall thereof for coupling RF energy thereto. A field effect electron emitter 16 is disposed within the cavity in electrical contact with one surface thereof and positioned for directing electrons substantially along the longitudinal axis of tube 10 toward a collector assembly 18. An accelerating anode 20 is disposed adjacent to cavity 12 between fieldeffect emitter 17 and collector 18 for developing the electric field between anode 20 and emitter l6 during operation of the tube. Additional coaxially disposed cavity resonators 22, 24 and 26 are serially disposed between anode 20 and collector 18 with a drift tube 28 disposed respectively therebetween. Tuning diaphragms 30 are positioned along the outer circumference of each cavity for fine tuning the respective cavities to a common frequency of oscillation, as is done in.

prior art tubes. A focus coil assembly 32 encompasses the cavity and drift tube assembly for maintaining the axial direction of flow of the electron beam. Cavity 26 functions as the output cavity in the preferred embodiment. having an output iris 34 thereof coupledto an ouput window 36 for coupling microwave energy received thereby to load circuitry.

As shown in FIG. 2, the field effect electron emitter 16 of cavity 12 has a backing plate 42 electrically in common with a wall 43 of the cavity. The emitter protrudes into the cavity in the capacitive area thereof with emitting rods 44 disposed for emitting electrons from respectiveends thereof toward the grid like opening 46 within the opposite cavity wall 48. When wall 48 3 is negative with respect to the wall 43 and the field effect emitter, no electrons flow from the field effect emitter. As shown, with wall 48 of the cavity positive with respect to surface or wall 43, field effect emitter 16 is negative and the positive potential on wall 48 adds to the potential developed between anode 20 and cavity wall 43 to provide the additional potential difference necessary for emission, allowing the field effect emitter to emit electrons. Thus, when the potential between grid like opening 46 and emitter 16 reaches a value determined by the particular tube design parameters, electrons flow from emitter 16 through screen 46 and into the remainder of the tube assembly. The particular tube design parameters, at which electron flow occurs, includes the spacing between screen 46 and emitter 16, the number of emitting fibers per unit area, and the electric field between the anode and emitter.

During operation, an input signal is applied to input loop 14 of input cavities 12. This signal causes the input cavity to oscillate which in turn causes the polarity of the potential across the cavity to periodically change. During the half cycle the field effect emitter potential is negative with respect to side 48 and electrons flow from the emitter through the cavity. Thus, the emitter emits electrons in bunches only during alternate half cycles of the input frequency on loop 14. Each bunch of electrons is accelerated by anode 20 to the desired velocity. The bunches, evenly spaced apart, enter the resonant cavity 22 where they are bunched to a greater degree. The bunch then moves through drift tube 28, through a third cavity 24, and through another drift tube 28 into the output cavity 26. Microwave energy is extracted from the electron bunch at the output cavity. Microwave energy is coupled through the output iris and the output window to a load such as an antenna. The electrons emerging from output cavity 26 are collected at collector 18 as in conventional tubes.

FIG. 3 provides a simplified comparison between prior art klystron operation (FIGS. 3A and 3B) and the improved beam tube operation (FIGS. 3C and 3D). With no input signal in the prior art klystron an unmodulated electron beam is generated as in A. With an input loop signal, bunching takes place as shown in B. No electron flow is shown in C, indicative of the improved beam tube operation with no input loop signal; and the electron beam is shown in D tobe generated in bunches within the input cavity in response to the input loop frequency.

A primary difference in operation of the improved microwave beam tube as compared with the conventional klystron is that separate bunches of electrons are produced in the input cavity rather than using the input cavity to bunch a continuous stream of electrons as in the prior art. As shown in FIG. 3, this results in short compact groups of electrons which allows either more compact groups of electrons to be provided for the same number of intermediate cavities as used in prior art klystrons or will allow elimination of some intermediate cavities while achieving the same results. The more compact bunching of electrons by the improved microwave beam tube and the elimination of stray electrons between bunches results in a greater amount of energy that can be coupled from the electron bunch in the output stage, as well as reducing output noise levels. Thus, electron emission is achieved within the input cavity as a function of the microwave frequency present on the cavity input loop, providing a cavity electron emission system that improves the output energy level.

Advantages of the improved microwave beam tube include reduction of weight and size of the beam tube due to more efficient design and elimination of auxillary components such as heater, heater transformer, electron gun assembly and associated control circuits. Because of the improved coupling resulting from more compact electron bunches the overall efficiency of the device is greater than prior art klystrons. There is no warm-up time required since the device is capable of instant response immediately upon input signal application. In the event that the input signal is lost for some reason, the apparatus is not harmed by an electorn beam since no electron beam can be generated unless the input signal is present in the input cavity. Conventional beam tubes require special protection circuits for the tube in the event of input signal loss to protect the tube from the continuous beam of electrons which is present regardless of whether an input signal is coupled to the input cavity or not.

A preferred embodiment of the invention has been chosen for purposes of illustration and description. The preferred embodiment illustrated is not intended to be exhaustive nor to limit the invention to the precise form disclosed. It is chosen and described in order to best explain the principles of the invention and the application thereof in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications as are best adapted to the particular use contemplated. It will be apparent to those skilled in the art that changes may be made in the form of the structure disclosed without departing from the spirit of invention as set forth in the disclosure. It is therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. Accordingly, it is desired that the scope of the invention be limited only by the claims appended hereto.

We claim:

1. In a microwave beam tube having an electron collector and a series of resonant cavities coaxially aligned for coupling an electron beam sequentially therethrough for obtaining velocity modulated output energy therefrom, the improvement of an input cavity electron emission system for providing periodic bunches of electrons emitted into the .beam path and comprising: an input cavity resonator coaxially aligned with said resonant cavities and having an input loop for receiving alternating microwave input signals; a field effect electron emitter disposed for periodically releasing electrons into said input cavity toward said collector in response to said alternating microwave energy developed within said cavity, said emitter comprising a plurality of parallel, insulated, emitting fibers per unit area having a conductive backing plate in common with one wall of said input cavity, said plurality of insulated emitting fibers are several million parallel fibers per square centimeter forming a planar emitting surface for emitting electrons at prevailing ambient temperatures; an accelerating electrode, said accelerating electrode being coaxially aligned between said input cavity and a first cavity of said coaxial resonant cavities for developing an electric field between the emitter and electrode for directing electron beam flow from said emitter therethrough', and an electron drift space separating each of said coaxial resonant cavities from adjacent of said cavities. 

1. In a microwave beam tube having an electron collector and a series of resonant cavities coaxially aligned for coupling an electron beam sequentially therethrough for obtaining velocity modulated output energy therefrom, the improvement of an input cavity electron emission system for providing periodic bunches of electrons emitted into the beam path and comprising: an input cavity resonator coaxially aligned with said resonant cavities and having an input loop for receiving alternating microwave input signals; a field effect electron emitter disposed for periodically releasing electrons into said input cavity toward said collector in response to said alternating microwave energy developed within said cavity, said emitter comprising a plurality of parallel, insulated, emitting fibers per unit area having a conductive backing plate in common with one wall of said input cavity, said plurality of insulated emitting fibers are several million parallel fibers per square centimeter forming a planar emitting surface for emitting electrons at prevailing ambient temperatures; an accelerating electrode, said accelerating electrode being coaxially aligned between said input cavity and a first cavity of said coaxial resonant cavities for developing an electric field between the emitter and electrode for directing electron beam flow from said emitter therethrough; and an electron drift space separating each of said coaxial resonant cavities from adjacent of said cavities. 